This invention relates generally to test devices for nuclear imaging systems, such as Anger camera systems used in radiology.
There are a multitude of methods available for evaluation of camera performance in nuclear imaging systems. These methods evaluate a number of parameters including uniformity, linearity (X and Y), spatial resolution, count efficiency, and image distortion. In the majority of cases, these evaluations are made under other than clinical conditions.
The more common use of phantoms constructed utilizing lead bars or holes drilled in lead (standard bar phantoms, Smith orthagonal hole phantoms) provide high input contrast to the detector assemblies. These methods commonly use 1/4 in. (0.635 cm) thick lead representing 28 half value layers for 140 kev photons, or 1/4 in. (0.635 cm) powdered tungsten representing 32 half value layers for 140 kev photons. Although the fidelity increases as bar width and interspacing decreases, the lack of a scattering media and the presentation to the detector of a high contrast input, tend to represent other than the clinical problem (namely, the detection of a focal void within an activity distribution).
Line spread function measurements made with line sources placed in air or scattering media, are difficult to perform and do not reflect the clinical problem. The most popular technique involving determination of the width of the curve at half the maximum value (FWHM), does not reflect the effect of scattering medium as deterioration of image quality due to this clinical problem rarely is seen above the 50% level in this measurement.
Results of a modulation transfer function measurement yields data relating primarily to the spatial resolution performance on a sinusoidal distribution of radioactivity. Again, the modulation transfer function is difficult to perform, and the data does not reflect the response of the imaging device to the detector of voids under clinical parameters.
Contrast efficiency can be evaluated by utilizing the Rollo phantom. This phantom contains an inner core that has 16 interconnecting cells. The cells are filled with a radioactive solution. Four different sphere diameters at four different contrast levels are imaged within the scattering media. Images of the phantom obtained on different collimators for equal data accumulation times will reflect trade-offs between contrast efficiency and sensitivity offered by each collimator type. The principal disadvantage of the Rollo phantom is that it only offers evaluation of the central portion of the imaging device, unless multiple images across the detector assembly are obtained. The evaluations of linearity and image distortion over the entire detector assembly is not easily appreciated.
With the preceding information in mind, I have developed a phantom which evaluates the total system camera performance under clinical parameters. It allows for the subjective evaluation of spatial resolution and contrast efficiency with respect to depth and various target to background ratios in the presence of a scattering medium. In the interest of maximizing the output contrast visibility of low input contrast lesions by the photographic system, the utility of the phantom described is discussed.
The intrinsic spatial resolution specifications provided by camera system manufacturers usually reflect the most favorable experimental circumstances. Intrinsically, current models of Anger camera systems are capable of resolving high contrast objects separated by 2.5 to 3.0 mm. in the energy region of 140 kev. With hopes of providing a more practical evaluation of system resolution, the phantom described herein was designed to simulate clinical parameters. The phantom permits the subjective evaluation of spatial resolution at depths, in the presence of scattering medium and varied input contrast.